Microelectronics packages called "multi chip modules" (MCM) can be constructed with semiconductor dice. Typically the dice are in a bare or unpackaged form. Non-conventionally packaged dice such as chip scale packages can also be used to construct multi chip modules. With a multi chip module, a number of dice are attached to a printed circuit board or other substrate and electrically connected to form various circuits and electronic devices.
For example, one type of multi chip module is a single in line memory module (SIMM). With a SIMM, several memory devices such as DRAMs are mounted on a printed circuit board having an edge connector. The SIMM is designed to plug into a computer socket in electrical communication with address, data and power supply buses for the computer.
One reason for the increased use of multi chip modules is increased system performance. In particular, the integrated circuits on the dice can be operated with lower resistance and parasitic capacitances. This is largely a result of decreasing the interconnection length between the dice. In addition, overall system performance is improved because the input/output ports can be configured to access the whole module, which can be organized to reduce signal delays and access times. The power requirements are also reduced due to a reduction in the driver requirements.
Typically the dice are mounted on a substrate having an interconnect pattern formed using a process such as screen printing. Different techniques are used for mounting the dice to the substrate and for providing interconnection and termination of the unpackaged dice. These techniques include wire bonding, tape automated bonding (TAB), micro-bump bonding and flip chip bonding.
For example, with flip chip bonding, each die is mounted circuit side down, and bond pads on the die are bonded to corresponding connection points on the substrate. Flip chips are formed similarly to conventional dice but require additional process steps to form solder bumps on the bond pads. The solder bumps are typically formed with a height of from 25 .mu.m to 75 .mu.m. The solder bumps separate the dice from the substrate and minimize the physical contact between the dice and substrate.
One important consideration in fabricating multi chip modules is the electrical connection between the bond pads on the dice and the connection points on the substrate. It is important that these electrical connections be formed with a low resistivity. Additionally, it is preferable that each electrical connection be formed with a minimum amount of damage to the dice and particularly to the bond pads of the dice.
Another important consideration in fabricating multi chip modules is the effect of thermal expansion on the electrical connections. If the dice and substrate expand by a different amount, stress may develop at the connection points and adversely effect the electrical connections. Stresses from thermal expansion can also lead to damage of the dice and substrate.
Yet another consideration in the fabrication of multi chip modules is that the size of semiconductor dice and the size and spacing of the bond pads on the dice have become smaller. This makes mounting and interconnecting of the dice on a substrate more difficult. This is especially true with components in which it may be necessary to integrate a large number of dice onto a single substrate.
In view of these and other problems associated with fabricating chip modules such as multi chip modules and memory modules, improved fabrication processes are needed in the art. The present invention is directed to a method for fabricating chip modules that is low cost and suitable to volume manufacture. In addition, this method provides an improved chip module characterized by a low resistance and reliable permanent electrical connection between the die bond pads and conductors on a supporting substrate.